Learn what dosimeters measure, what radiation units mean, and how to interpret readings to make informed shelter and movement decisions after a nuclear event.
After a nuclear detonation or radiological incident, one of the most practically valuable things a prepared civilian can possess is a radiation detector. The ability to measure the radiation environment — rather than guessing at it — enables rational decision-making: when it is safe to move, whether shelter is effective, whether a food or water source is contaminated, and what cumulative dose you and your family are accumulating.
This guide explains the types of detectors available, the units of measurement and what they mean practically, how to interpret readings, and how to keep a dose log for medical purposes.
It is important to distinguish between different measurement functions:
Dose rate meters (Geiger counters and most survey meters) measure the rate at which radiation is currently hitting the detector — expressed in dose units per hour (e.g., mSv/hr, mR/hr, or μSv/hr). This tells you the intensity of the current radiation field.
Integrating dosimeters (personal dosimeters) accumulate and display the total dose received since the dosimeter was last reset — expressed in total dose units (mSv, mR, rem). This tells you how much radiation you have personally received.
Contamination meters (specific Geiger-Müller tubes) detect the presence of radioactive contamination on surfaces or skin — useful for checking whether a person or object carries radioactive particles.
A practical emergency kit should ideally include at least one dose rate meter and personal dosimeters for each family member.
The most widely available type. A Geiger-Müller tube detects ionising events and produces an audible click and/or digital count.
Reads: Dose rate (displayed as counts per minute — CPM — or converted to dose rate units). Most quality consumer units show μSv/hr or mR/hr.
Strengths: Widely available; detects alpha, beta, and gamma (depending on tube type); can check for surface contamination; audible indication is useful.
Limitations: Most consumer Geiger counters saturate (give inaccurate readings) at very high dose rates typical of early fallout environments. Check the upper range of your specific device.
Approximate cost: $150–$600 for quality devices. Avoid very cheap units — calibration accuracy varies significantly.
A self-reading dosimeter shaped like a pen. An electrostatic charge is set at zero; ionising radiation discharges it over time. Read by holding up to light and looking through the eyepiece.
Reads: Cumulative dose (typically in mR or mSv) up to its rated maximum.
Strengths: Inexpensive; no batteries; simple to read; can be worn on person.
Limitations: Must be reset before use; can discharge due to physical shock (false high reading); limited range; does not show dose rate.
Approximate cost: $20–$80 per unit. Military surplus pen dosimeters are widely available.
Electronic devices that integrate dose over time and display cumulative dose and sometimes dose rate.
Reads: Cumulative dose and/or current dose rate; typically gamma and X-ray.
Strengths: Easy to read; can alarm at preset dose thresholds; data logging in some models.
Limitations: Battery dependent; more expensive; may not detect alpha or beta well.
Devices that plug into a smartphone's audio jack or lightning/USB-C port and use an app to display readings.
Reads: Gamma and X-ray dose rate, typically.
Strengths: Inexpensive ($30–$150); app interface is user-friendly; some allow logging.
Limitations: Phone battery dependency; accuracy varies; most are sensitive instruments for low-level measurement rather than high-range emergency instruments; calibration is inconsistent across devices.
In a declared nuclear emergency, government emergency management agencies may distribute dosimeters to civilians and emergency responders. The KFM (Kearney Fallout Meter) can be improvised from household materials according to FEMA guidance and is a credible option where purpose-built devices are unavailable.
The field of radiological measurement uses multiple unit systems, which causes significant confusion. Here is a practical guide:
| Unit | Definition | Practical Use |
|---|---|---|
| Sv/hr (Sievert per hour) | SI unit; accounts for biological effectiveness | International standard; used in medical and most modern equipment |
| Gy/hr (Gray per hour) | Absorbed energy per unit mass; does not account for biological effectiveness | Used in physics; for gamma/X-ray, 1 Gy = 1 Sv |
| mSv/hr (millisievert per hour) | 1/1000 of Sv/hr | Common for consumer devices; background ≈ 0.1–0.3 μSv/hr |
| μSv/hr (microsievert per hour) | 1/1,000,000 of Sv/hr | Consumer device display; normal background ≈ 0.1–0.3 μSv/hr |
| R/hr (Roentgen per hour) | Older unit; measures ionisation in air | Common in older US devices; for practical purposes, 1 R ≈ 1 rad ≈ 0.01 Sv |
| mR/hr (milliroentgen per hour) | 1/1000 of R/hr | Common in US consumer and surplus devices |
| CPM (Counts Per Minute) | Raw detector count; not directly a dose unit | Must be converted using device-specific conversion factor |
Converting CPM to μSv/hr: The conversion factor varies by detector tube type. Common Geiger-Müller tubes (e.g., SBM-20) convert at approximately 150–175 CPM = 1 μSv/hr. Check your specific device's documentation.
| Unit | Definition | Notes |
|---|---|---|
| mSv (millisievert) | International standard cumulative dose | 1 mSv = typical annual background exposure for many countries |
| Sv (sievert) | 1,000 mSv | Used for high doses (radiation therapy, significant nuclear exposure) |
| rem | Older unit: 1 rem = 0.01 Sv = 10 mSv | Common in older US dosimeters |
| mrem | 1/1000 of rem; = 0.01 mSv | Very common in US personal dosimeters |
| rad | Absorbed dose; for gamma ≈ rem | Older unit |
Understanding what is "normal" is essential for interpreting readings meaningfully.
Normal background radiation varies by location:
Dose rate thresholds for decision-making:
| Dose Rate | Interpretation | Action |
|---|---|---|
| < 1 μSv/hr | Background or near-background | Normal — no action needed |
| 1–10 μSv/hr | Elevated — above normal background | Note and monitor; may indicate local contamination |
| 10–100 μSv/hr | Significantly elevated | Minimise outdoor time; check shelter effectiveness |
| 0.1–1 mSv/hr (100–1,000 μSv/hr) | High — fallout zone possible | Shelter; do not go outdoors except on emergency basis |
| 1–10 mSv/hr | Very high — active fallout zone | Strict shelter; cumulative dose accumulating rapidly |
| > 10 mSv/hr | Extreme — severe fallout zone | Absolute shelter; hourly monitoring; consider evacuation if reading persists |
Cumulative dose thresholds with medical significance:
| Cumulative Dose | Medical Significance |
|---|---|
| < 100 mSv | No clinically observable acute effects; minor statistical cancer risk increase |
| 100–500 mSv | Possible very early ARS symptoms at upper end; blood count changes detectable |
| 500–1,000 mSv (0.5–1 Sv) | ARS threshold; nausea likely; increased infection risk |
| 1–2 Sv | Mild ARS; most people recover with medical care |
| 2–6 Sv | Moderate to severe ARS; approximately 50% mortality at 4.5 Sv without treatment |
| > 6 Sv | Severe ARS; majority fatal without intensive medical care |
Is my current shelter effective? Take a reading outdoors (brief exposure) and immediately inside your shelter. The ratio gives you an empirical protection factor. If outdoor reading is 500 μSv/hr and indoor reading is 5 μSv/hr, your shelter is providing a PF of 100 — excellent.
Is the dose rate decreasing? Record readings every hour. The dose rate should be declining over time as fallout decays (the 7-10 rule: every 7-fold increase in time = 10-fold decrease in dose rate). If readings are increasing, your shelter may be accumulating fallout or you may be in a particularly heavy deposition zone.
Should I move to a better shelter? If your current shelter shows a very high indoor dose rate and a substantially better shelter is close, the brief exposure during movement may be worthwhile. Calculate approximate movement dose: (distance in metres / walking speed) × dose rate = exposure during movement. Compare to the dose saved per hour in the better shelter.
When is it safe to briefly go outside? Official guidance should drive this decision. In the absence of official guidance: if dose rate has declined below 1 mSv/hr (1,000 μSv/hr) and it has been at least 24 hours since the detonation, brief outdoor movement for critical purposes (water, medical emergency) becomes more defensible, but should still be minimised.
WARNING: Do not take dose rate readings as the sole basis for leaving shelter early. Fallout decay is not uniform. Dose rates can spike as new fallout deposits arrive with shifting wind. Always prefer official emergency broadcast guidance over instrument readings alone.
Several smartphone apps claim to detect radiation using the phone's camera sensor (without any external hardware). These are unreliable and should not be used for emergency decisions. Smartphone camera sensors are mildly sensitive to high-energy gamma but cannot be calibrated and have no consistent sensitivity.
Apps used with proper external hardware sensors (plugged-in Geiger-Müller tubes) can be useful for data logging and visualisation, but the hardware quality drives the measurement quality.
A dose log records your cumulative radiation exposure over time and is valuable medical information if you seek care for potential ARS or want long-term monitoring.
What to record:
Keep this log on paper — do not rely on phone battery.
For medical staff: The onset time of any nausea or vomiting is a particularly important indicator of dose received. Note the exact time symptoms first appeared relative to the detonation.
| Measurement | Unit | Background | Shelter-in-Place Threshold | Severe Hazard |
|---|---|---|---|---|
| Dose rate | μSv/hr | 0.1–0.3 | > 100 | > 10,000 (10 mSv/hr) |
| Dose rate | mR/hr | 0.01–0.03 | > 10 | > 1,000 |
| Cumulative dose | mSv | ~0.2/day background | — | > 500 acute |
| Cumulative dose | rem | ~0.02/day background | — | > 50 acute |
A radiation dosimeter cannot make you safer by itself — it only provides information. The value is in using that information to make better decisions about shelter, movement timing, and medical attention. For anyone in a region at risk from nuclear events, a quality Geiger counter and personal dosimeters are among the most practical preparedness investments available.
Take Radiation Dose Monitoring & Dosimeters with you — no internet needed when it matters most.
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